This invention relates to a process and apparatus for measuring substances in a body of liquid, e.g., contaminants in waste water discharged from chemical manufacturing plants. The present process is referred to as sparging, because gas bubbles are used to remove vaporized contaminant samples from the liquid. In systems of the type disclosed, the materials removed from the liquid are subjected to analysis by an instrument capable of measuring the concentration of specific chemicals in the gas phase as a function of time. This instrument could be in infrared spectrometer used in conjunction with a gas cell or a mass spectrometer.
Application Ser. No. 662,933, filed Mar. 1, 1991, having the same assignee as the present application, discusses in detail the purposes and functions of a novel sparging IR system, and refers to a prior sparging IR system developed by the duPont Corporation.
A deficiency in both of the sparging IR systems disclosed in the previous application is the possible inability to correctly measure the concentrations of some contaminants. The problem is due to depletion of constituent traces before their respective vapor pressures have reached equilibrium with the concentrations of the corresponding solutes in the waste water. This will lead to an unduly low measurement of any constituents which have been significantly depleted before the IR analysis occurs.
In application Ser. No. 662,933 this problem was recognized, as indicated by this quotation: "A further aspect of the present invention is the use of increased liquid flow rates to prevent or reduce depletion of contaminants which are only slightly soluble, and are therefore likely to be inaccurately measured, if they are significantly depleted during the sparging process. Having a sufficiently high ratio of liquid flow rate to gas flow rate is useful in preventing contaminant depletion."
Experience has shown that two of the values which control the contaminant concentration measurements vary considerably with operating conditions. One such value is the equilibrium vapor pressure (designated "P.sub.o ") of the pure substance being measured. The other such value is the solubility in water (designated "S") of the substance being measured. The value P.sub.o is temperature dependent, and the value S is both (a) highly temperature dependent, and (b) influenced by the presence of other chemicals in the water.